Managed switches permit some of the following configurations:
* Turn some particular port range on or off
* Link speed and duplex settings
* Priority settings for ports
* MAC filtering — and other types of "port security" features which prevent MAC flooding
* Use of Spanning Tree Protocol
* SNMP monitoring of device and link health
* Port mirroring (also known as: port monitoring, spanning port, SPAN port, roving analysis port or link mode port)
* Link aggregation (also known as bonding, trunking or teaming)
* VLAN settings
* 802.1X network access control
* IGMP snooping
Link aggregation allows the use of multiple ports for the same connection achieving higher data transfer speeds. Creating VLANs can serve security and performance goals by reducing the size of the broadcast domain.
Unmanaged switches do not have the ability to make such changes.
Most managed switches provide a handy web interface where you can see whats going on.
Most unmanaged switches do not have that interface, and just sit in a closet.
Many companies make "Smart Switches" which are not fully managed switches but are not unmanaged. Such products, like those from Netgear are both reliable, easy to use, and relatively inexpensive.
Hope this helps!